1. Field of the Invention
The present invention relates to a pigment dispersion, and an ink composition, a curable composition and a curable ink composition each produced with the pigment dispersion. More specifically, the invention relates to a pigment dispersion containing a highly dispersed pigment and having high dispersion stability; an ink composition that contains the pigment dispersion, has high storage stability under high temperature conditions and is capable of forming high-quality images; a curable composition capable of being cured by an irradiation of active energy rays; and a curable ink composition that is produced with the curable composition and capable of forming high-quality images.
2. Related Art
Image recording methods for forming images on recording media such as paper and plastic sheets based on image data signals include electrophotographic methods, sublimation- and melting-type thermal transfer methods, and inkjet methods. In particular, inkjet methods are applicable to inexpensive devices and make it possible to form an image directly on a desired image-receiving portion of a recording medium in an ink jetting process. Therefore, inkjet methods are advantageous in that ink may be efficiently used, running cost is low, and high resolution images may be recorded by jetting very small droplets.
Inkjet methods make it possible to perform printing on non-absorbent recording media such as plastic sheets and metal plates as well as printing on plain paper. The significant challenge to inkjet methods is to increase printing speed and image quality, and inkjet methods are characterized in that drying of droplets after printing and the time required for curing have a significant effect on the image sharpness.
A recoding method using an inkjet recording ink composition capable of being cured by the application of active energy rays is one of the inkjet methods. In this method, ink droplets may be cured by irradiation immediately after printing so that sharp images may be formed.
Such a curable inkjet ink composition requires high pigment dispersibility for the formation of high resolution images with high color development. Concerning inkjet ink, it is also important to control the viscosity and surface tension of the ink, because fine ink droplets are jetted at high frequency from small orifices.
Making fine pigment particles are essential to impart vivid color tone and high coloring strength to ink compositions. In particular, an inkjet recording ink composition has to be produced with particles smaller than the thickness of the cured ink film to be formed with the ink composition, because the jetted ink droplets have a significant effect on the sharpness of the image and therefore the amount of the jetted droplets is kept small. As pigment particles are made finer for higher coloring strength, it becomes more difficult to disperse the particle, and aggregates are more likely to be produced. In addition, when a dispersing agent is added, the problem of an increase in the viscosity of the composition may also occur. The production of pigment aggregates or an increase in the viscosity of the ink composition has an adverse effect on the ink jetting property. Therefore, an ink composition containing pigment aggregates or having an increased viscosity is not suitable for use in inkjet recording.
An inkjet recording ink composition also has to have good heat-cycle resistance. An ink composition contained in a cartridge is heated so as to have a reduced viscosity during jetting but cooled during the non jetting period or storage. Therefore, it undergoes cycles of temperature change between heating and cooling. The temperature change also has an adverse effect on the pigment dispersibility and causes a problem in which the pigment dispersibility is decreased over time so that pigment aggregation or viscosity increase is more likely to occur.
Curing properties for the formation of sharp images and the physical properties of the cured film are also important factors of a curable inkjet ink composition. A curable inkjet ink composition should be more cured rapidly by irradiation so that the sharpness of the image may be ensured. Therefore, a pigment dispersing agent that may inhibit curing, a pigment dispersing agent that contains a solvent as a non-curable component, or a pigment dispersing agent that is not crosslinked with the cured film and may reduce the surface tackiness is not preferred, because it may cause a reduction in curing properties, blurring, or a reduction in productivity.
If such a compound is mixed, a so-called blocking problem may occur in which the printed surface adheres to the recording medium during the storage of stacked prints so that the printed surface of the recorded product and the recording medium are stained.
Therefore, there has been a demand for an ink composition that has sufficient fluidity, contains a finely and stably dispersed pigment and has good curing properties. Various dispersing agents are proposed to form stable pigment dispersion.
There are proposed an ink composition produced using a pigment derivative as a dispersing agent for improving the affinity to the pigment (see for example Japanese Patent Application Laid-Open (JP-A) Nos. 2003-119414 and 2004-18656), an ink composition produced using a specific pigment such as a phthalocyanine or quinacridone pigment and a basic group-containing polymer as a dispersing agent (see for example JP-A No. 2003-321628), an organic solvent-free ink composition containing such a dispersing agent as a poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and a specific monomer capable of dissolving the dispersing agent (see for example JP-A No. 2004-131589), an ink composition produced using, as a dispersing agent, a polyurethane having a pendant of a neutralized salt such as a carboxylate, a phosphate, a sulfonate or a quaternary ammonium (see for example Japanese Patent Application National Publication (Laid-Open) No. 2002-503746), a non-aqueous pigment dispersion composition produced using a graft copolymer of a nitrogen atom-containing, polymerizable, vinyl macromonomer (see for example JP-A No. 2007-277506), and a polyurethane dispersing agent containing a polymer including N-vinylamide, N-vinyllactam, vinyl, or ally-substituted, nitrogen-containing heterocycle and containing another polymer as a graft chain (see for example Japanese Patent Application National Publication (Laid-Open) No. 2002-526254).
The use of these pigment dispersing agents or ink compositions makes it possible to ensure fine dispersion of pigments and to have higher ink stability than the other conventional techniques. However, when they are used to form low-viscosity ink compositions such as inkjet ink compositions, the high-temperature stability of the resulting ink compositions is not sufficient, and when they are used to form curable ink compositions, the curing sensitivity or anti-blocking properties are insufficient.
There is also proposed a method of using an acrylic copolymer having a tertiary amino group and an atomic group of an organic pigment to produce an ink composition excellent in fluidity, film vividness, glossiness, and coloring strength (see for example Japanese Patent No. 2993088). However, such a technique has an insufficient affinity to the ink medium and therefore further stability is required.